The incidence of breast cancer carcinoma among women continues to increase, posing a serious health problem throughout the world. The mortality rate from breast cancer can be decreased significantly by early detection using the radiological mammography technique. With this technique the compressed breast is irradiated with soft X-rays emitted from an X-ray generating device and the modulated X-rays are detected with a radiographic X-ray conversion screen, also called intensifying screen, fluorescent screen or phosphor screen. The X-ray conversion screen comprises a luminescent phosphor which converts the absorbed X-rays into visible light and the emitted visible light exposes a silver halide film that is brought into contact with said X-ray conversion screen. After film processing, comprising the steps of developing, fixing, rinsing and drying, a mammogram is obtained which can be read on a light box.
No other field of medical radiology demands such a high level of image quality as mammography and the ability of the mammogram to portray relevant diagnostic information is highly determined by the image quality of the screen-film system. Image quality is manifested by a number of features in the image including sharpness, noise, contrast, silver image color and skin line perceptibility. It is common practice to set the amount of X-ray exposure so that the tissues on the inside of the breast are depicted at medium optical density values, i.e. in the optical density range from Dmin+1.0 to Dmin+2.5 (Dmin being defined as the base+fog density obtained after processing the unexposed film), and the diagnostic perceptibility of small, potentially malignant lesions in these tissues is highly determined by the contrast of the mammography film within said density range. A quantitative measure of the film contrast is the so-called average gradation, defined as the slope of the line drawn by connecting both points of the sensitometric curve of optical density vs. logarithmic exposure at which the optical density is equal to Dmin+1.0 and Dmin+2.5.
Conventional mammography films can roughly be classified in low and high contrast types according to the value of their average gradation as defined above. The low contrast type can be characterized by a relatively low average gradation ranging from 2.0 to 2.5 whereas the average gradation of the high contrast type may range higher than 3.0. Often, high contrast films are preferred because of the higher ability to detect tiny cancers deep in the glandular tissue of the breast. If the contrast is too high, however, it may preclude visualisation of both thin (i.e. the skin line) and thick tissues (i.e. the inside of the breast) in the same image due to lack of exposure latitude. Therefore, some radiologists prefer low contrast mammography films. When the contrast is low, skin line perceptibility is excellent, but then the chance of missing possibly malignant breast lesions is high. Thus a balance has to be found between contrast and exposure latitude and an example of this approach has been described in U.S. Pat. No. 5,290,655.
In order to extend the exposure latitude some manufacturers have introduced high contrast mammography films characterized by a higher maximum density (hereinafter referred to as Dmax) than conventional high contrast films, e.g. a Dmax equal to at least 3.7, preferably even higher than 4.0. However, a film characterized by a higher Dmax is only a minor improvement with regard to better skin line perceptibility, since the background density is too high for the skin line to be clearly visible. Indeed at optical density values above 3.5, the local gradient, i.e. the slope of the sensitometric curve must be very high in order to guarantee a reasonable perceptibility as described in the classic article `Determination of optimum film density range for rontgenograms from visual effect` by H. Kanamori (Acta Radiol. Diagn. Vol.4, p. 463, 1966). Nevertheless, mammography films with a higher Dmax are appreciated by a growing number of radiologists because of the wider dynamic range, i.e. the density range Dmax-Dmin of the mammogram.
Some improvements have been obtained by modifying the X-ray generating device, such as the scanning mammography system described in U.S. Pat. No. 5,164,976. These solutions however require the replacement of the conventional X-ray apparatus by a completely new system of a much higher technical complexity. Athough it remains difficult to obtain mammograms with high contrast and high Dmax that also clearly depict thin tissue such as the skin line of the breast a suitable solution has been offered as disclosed in EP-A 0 874 275. Therein a silver halide photographic material has been disclosed, having a total silver coverage from 6.0 to 9.0 g/m.sup.2 expressed as silver nitrate, wherein the emulsion layer(s) comprise(s) cubic or tabular silver halide grains and wherein the silver coverage of the tabular silver halide grains is lower than the silver coverage of the cubic silver halide grains. Said material is further characterized by a sensitometric curve of optical density as a function of relative logarithmic exposure (hereinafter referred to as logE), having an average gradation of at least 3.5 between the two points where the optical density is Dmin+1.0 and Dmin+2.5 (Dmin being defined as the optical density obtained after processing the unexposed film); a local gradient ranging from 0.7 to 1.8 and an optical density not higher than 3.5 at the point where logE equals SP+0.8 (SP being defined as the logE at which the optical density equals Dmin+1.0); and an optical density of at least 3.7 at the point where logE equals SP+1.3.
Maintaining the image quality constant is becoming another requirement of facilities performing mammography. Accordingly, quality control tests are executed on a regular basis in order to monitor the consistency of the performance of the X-ray equipment, the image receptors and the film processor. In order to minimize the influence of varying film processing time, temperature, chemistry and replenishment, a preferred mammography film requires a stable speed and contrast with regard to these processing parameters. In addition, there is a general trend in the field of radiology to shorten the film processing time and likewise in the field of mammography, being driven by intensified screening programs, the interest has focused on rapid access of mammograms.
As a consequence, mammography films are preferred which comprise silver halide crystals that can be processed rapidly and consistently in a dry-to-dry processing cycle of 90 seconds or less and therefore, most mammography films today comprise good developable cubic silver halide crystals. As described in EP-A 0 712 036 such cubic crystals show a stable speed and contrast upon varying processing parameters. Cubic emulsions however are characterized by a very high contrast, resulting in a poor skin line perceptibility.
Especially in rapid processing applications it is very difficult to obtain the desired low fog, high speed and high covering power simultaneously. Replacing cubic grain emulsions by tabular grain emulsions is in favour of getting a high covering power at moderate coating amounts of silver halide as has been demonstrated e.g. in U.S. Pat. No. 4,414,304. Disadvantages of tabular grains however are the lower cantrast than the contrast obtainable with cubic grains and residual coloration of the processed image, especially in short processing cycles, due to strong adsorption of huge amounts of spectral sensitizing dye(s) at the large specific surface area, characteristic for the said tabular grains.
Making use of a mixture of cubic and tabular grains or of a multilayer arangement of cubic and/or tabular grains as in EP-A's 0 874 275 and 0 770 909 respectively is more complex and less interesting from the point of view of reproducibility of the production process.